For resource saving and improved economics it is advantageous to reduce the amount of raw materials needed for the production of a specific type of product. In a mill annually producing 200 000 tons of paper board for liquid packaging, a reduction of the amount of raw materials needed for 1 litre packages by 1% would make it possible to pack for example 70 million litres of milk products more per year without increasing the amount of raw materials used.
In Sweden this corresponds to the yearly consumption of milk in a city of the size of Gothenburg.
However, savings of raw material should desirably be achieved without having to compromise with the quality requirements of the product.
Important properties of coated paperboard for formed articles are sufficient stiffness measured as bending force, good forming properties including low susceptibility to crack formation at folding, adequate surface for printing and adequate surface gloss. All of these required properties vary with the specific type of formed article which is to be produced from the coated paperboard.
A conventional way of producing board with high stiffness is to use a fibermatrix with three or more layers where the middle layer gives bulk and the two outer layers have a high elasticity modulus or high tensile stiffness. However, there are also coated board with high stiffness made from only two fiber layers.
In the production of coated paperboard it is known that the calendering operation together with the coating contributes to a good printing surface. However, at the same time the stiffness is to a certain extent reduced. In order to achieve an adequate surface for printing a calendering operation has been performed to reduce the surface roughness before the paperboard is subjected to a coating operation. Depending on the type of final product to be produced sometimes there is also performed a calendering operation after the coating operation to further improve the surface and increase the gloss to the desired level.
There are mainly two types of heatable or non-heatable calenders which are in use in paper mills today, namely hard nip calenders which have steel rolls, and soft nip calenders which have steel rolls where the counter rolls have a softer cover. The nip lengths in these soft nip calenders are typically approximately 1 cm.
A new type of calender is disclosed in Pulp & Paper International (PPI), May 1994, page 36. Surface properties of an uncoated board grade were reported to be improved with only minor loss of stiffness (bulk). The new calender is constructed to have a relatively soft elastic moving belt supported by a glide body or roll instead of the covered roll in a conventional soft calender. The elongated nip contour in combination with the soft elastic belt is reported to yield uniform specific pressures and nip lengths which typically would be four to six centimeter. This new type of calender has been named supersoft calender in said article, and said calender could be used in the present invention where a heatable calender with soft extended nip is required.
In the production of paperboard for formed articles the calendering operation has up to now been performed either before or both before and after the coating operation.